1. Field of the Invention
The present invention relates to a method of digital quadrature amplitude modulation using spectrum-shaping digital filters wherein sampling sequences consisting of two carrier oscillations residing at 90.degree. relative to one another are available.
2. Description of the Prior Art
In terms of basic concept, methods for quadrature amplitude modulation (QAM) have been disclosed in the publication NTG Fachberichte (1980), pp. 81-85. Digital, spectrum-covering low-pass filters are also employed in this reference. The digital/analog conversion subsequently occurs and the quadrature amplitude modulation occurs on an analog basis. This also applies, analogously, to the receiving path.
It should be further taken into consideration that circuits that meet the so-called Nyquist condition are required for the resolution of a number of telecommunications transmission problems. This condition is defined per se in that appertaining circuits are required when, for example, it is a matter of transmitting pulses, without intersymbol noise (i.e. pulses without crosstalk) or, respectively, of converting video frequency bands, whereby the frequency zero and a vestigial sideband must also be transmitted. Possibilities for the synthesis of Nyquist filters are specified for processing analog signals, for example, in the periodical "Frequenz", 1973, Vol. 27, pp. 2-6. In terms of basic concept, these circuits amount to the fact that realizing so-called strict diplexers wherein filters having a so-called self-reciprocal characteristic function are employed. In the meantime, so-called digital technology is also employed in telecommunications transmission systems and, therefore, the problem also arises of creating selection devices therefore, i.e. circuits and networks for which the term digital filters has become standard. Possibilities for the realization of such digital filters, for example, are set forth in rather general terms in the book by Temes and Mitra, entitled "Modern Filter Theory and Design", John Wiley & Sons, 1973, particularly on Pages 505-557. So-called wave digital filters comprise a special species of these filters. Such circuits are disclosed, for example, in the German Letters Pat. No. 2,027,303 or, respectively, in the periodical "AEU", Vol. 25 (1971), pp. 79-89. Wave digital filters have the advantage that analog reference filters can be directly digitally simulated.
As already mentioned above, the quadrature amplitude modulation in the initially-mentioned article (NTG Fachberichte, pp. 81-88) is carried out on an analog basis.